In an injection molding machine, a die casting machine or the like, the pair of molds therein must be kept in a closed state when a molding product is being molded. In order to maintain the molds in the closed state against a greater inner pressure produced within the molds, a mold clamping operation to press the pair of molds towards each other by strong forces from the opposite sides is necessary. For this reason, in a molding device in the injection molding machine or the like, a mold clamping device for maintaining the molds in the closed state during the molding operation is provided together with an injection device for injecting a raw molding material in a molten state.
Such a conventional mold clamping device is known, for example, from JPA-61-193821, in which a nut member secured to a movable platen is provided with a ball screw, which is driven in a forward rotation or a reversing rotation by means of a servo motor while the torque of the servo motor is controlled so that the compressibility of the ball screw is controlled, thereby enabling the mold clamping force to be controlled.
However, since this type of mold clamping device attempts to provide a greater force, which is necessary to clamp the molds by the servo motor, there is a problem of requiring a motor with a greater horsepower.
To deal with this problem, as disclosed in, for example, JPB-46-25262 or JPA-61-154822, mold clamping devices are proposed in which the clamping of the metal molds is performed by the use of a magnetic attracting force.
However, the conventional mold clamping device in which the clamping of the metal molds is performed by the use of a magnetic attracting force has a disadvantage in that change in the thickness of the metal molds results in change either in the relative position of the magnetic means used to produce the magnetic attracting force or in the relative position between the electromagnet and the attracted part, thereby providing a mold clamping force only at a specific position. Thus, in such a type of conventional mold clamping device, there are further disadvantages in that an operation of adjusting the thickness of the molds is necessary every time the metal molds are changed and, at the same time, the construction of the device becomes complicated and changes of the metal molds are not easy.